The present invention relates to a focal type(or focusing) electromagnetic ultrasonic transducer for detecting flaws on a surface of an object made of a conductive material, and flaws and internal deteriorations of an object, caused by an applied load and/or heat. The present invention also relates to an ultrasonic flaw detection method with the focal type electromagnetic acoustic transducer.
As one of the prior art techniques of an ultrasonic flaw detection, Japanese Patent Public Disclosure (Kokai) No. 1-127950 (1989) discloses a focusing electromagnetic acoustic transducer. This focusing electromagnetic acoustic transducer comprises an array of a plurality of permanent magnets, which are arranged on a line sequentially from thicker ones to thinner ones, and a coil wound around the magnet array. The alternating magnets of the array have different polarities from each other and portions of the magnets having such different polarities form a plane. The coil passes along the plane containing the polarities of the magnet array and the magnets are bound into a bundle.
Since the focusing electromagnetic acoustic transducer has magnets arranged from the thickest to the thinnest in a line such that respective adjacent magnets have different polarities from each other, wave fronts of acoustic waves in the depth direction of an object become in focused and are thus capable of improving a temporal resolution and facilitating the detection of flaws within an object.
The focusing electromagnetic acoustic transducer configured as described above requires magnets to have a reduced thickness in order to generate acoustic waves at high frequencies to thereby provide excellent flaw detecting capabilities. However, with the use of such magnets having smaller thickness, the transducer is more susceptible to a lift-off influence, and a magnetic flux of a magnet intrudes into an immediately adjacent magnet and thus an effective magnetic field may not be formed. Accordingly, appropriate actions of the electromagnetic acoustic transducer may be prevented by making the magnets thinly.
Further, the focusing electromagnetic acoustic transducer generates SH waves. The SH waves induce a problem that they have a wide radiation distribution so that a sharp directivity cannot be realized even if the thickness of the permanent magnets is varied to narrow down the focus.
Japanese Patent Public Disclosure (Kokai) No. 1-248052 describes an ultrasonic transducer. The transducer also generates SH waves and hence gives rise to the same problem as above.
Each of Japanese Patent Public Disclosure (Kokai) Nos. 63-305245 and 64-10168 describes an elevation type electromagnetic acoustic transducer which has coils with a fixed pitch and flowing currents at different frequencies through the respective coils. The elevation type electromagnetic acoustic transducer, however, requires complicated signal processing and is susceptible to producing disturbed acoustic waves within a material or object.